Pins are common machine elements typically employed to ensure accurate positioning of parts or to transmit relatively small shear forces. Many types of pins have been developed for a variety of applications, including cotter pins, spring pins, straight pins, grooved pins, taper pins and knurl pins. Of these, the invention relates most closely to knurl pins and grooved pins employed as positioning pins. In a typical positioning pin application, first and second parts to be positioned define holes at complementary positions. Pins are installed in the holes of the first part. The pins protrude from the first part to align second part relative to the first part by engaging the holes of the second part. Ideally, the pins are centered in both sets of holes and exert some frictional force on the holes.
Knurl pins are pins having an outside surface that is deformed into a plurality of straight or helical knurls. The knurled surface portion of the pin includes a series of crests and troughs, with pin material from the troughs displaced to the crests to define an expanded outside diameter for the pin. A common knurled surface is one in which the crests have a width (measured perpendicular to the crest length) that is approximately 10% of the pitch of the knurl pattern. This shape of knurl is relatively sharp. Typically, the knurl pin material is harder than the host material, allowing the crests of the knurled outside surface to cut into the softer host material to provide enhanced retention of the pin in the host. An exemplary prior art knurled pin is shown in FIG. 1.
A grooved pin enhances retention force by disrupting the outside surface of a straight solid pin with one or more V-grooves. Pin material adjacent the V-groove is displaced upwardly and outwardly to each side of the groove, forming a raised portion or flute extending alongside the groove. The crests of the flutes provide an expanded diameter a few thousandths larger than the nominal diameter of the pin. When a grooved pin is driven into a drilled hole of a predetermined diameter, the raised portion of the pin is supposed to be forced back into the groove where it resiliently exerts a radially outward retaining force against the inside surface of the hole in the host. The above-described theoretical operation of a grooved pin is rarely achieved in practice. In soft host materials, the grooved pin crests dig into the host, while in hard host materials, the crests are scraped off as further discussed below. A sectional view of an exemplary prior art grooved pin is shown in FIG. 2.
Knurl pins and grooved pins are most successful when the host material is no harder than the pin material. In assemblies where the host material is significantly harder than the pin, high profile raised features with small sectional areas tend to be sheared away as the pin is driven into the host as shown in FIGS. 7 and 8. As a result, retention force and other measures of pin performance are severely compromised. This is particularly the case in assemblies that are repeatedly taken apart and reassembled.
There is a need in the art for a solid pin that provides reliable and repeatable press-fit retention and positioning in a host having hardness greater than the pin.